Charged particle beam apparatus, and method of controlling the same

ABSTRACT

A charged particle beam apparatus includes a field emission electron source, electrodes for applying an electric field to the field emission electron source, and a vacuum exhaust unit for keeping the pressure around the field emission electron source at 1×10 −8  Pa or less. The apparatus uses electron beams emitted to have an electron-beam-center radiation angle of 1×10 −2  sr or less, and uses the electric current thereof, the second order differentiation of which is negative or zero with respect to time, and which reduces at a rate of 10% or less per hour. A heating unit is provided for the field emission electron source, and a detection unit is provided for the electric current of the electron beam. The field emission electron source is repeatedly heated to keep the electric current of the electron beam to be emitted, at a predetermined value or higher.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/999,075, filed Dec. 15, 2010, which is a §371 national stageapplication of PCT/JP2009/002620, filed Jun. 10, 2009, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a charged particle beam apparatus suchas an electron microscope having a field emission electron gun (FEG),and more particularly, to a charged particle beam apparatus forstabilizing current of electron beams and a method for controlling thesame.

BACKGROUND ART

When a positive voltage is applied from a facing electrode to a metalprocessed to be in a needle shape, an electric field is concentrated ona front end of the metal. When a strength of the electric field is about10⁷ V/cm, free electrons in the metal may escape a potential barrier ofthe surface due to the tunneling effect and be externally emitted. Thisphenomenon is referred to as a field emission.

An apparatus for acquiring electron beams by the field emission isreferred to as a field emission electron gun (FEG). As for an electronsource of the FEG, a tungsten beam formed of a single crystal that issharpened to be in a needle shape is generally used and is used at roomtemperature. The electron source and an extraction electrode facing theelectron source are installed in a vacuum container and an electron isemitted by applying an extraction voltage to the electron source and theextraction electrode. The emitted electron is accelerated at a highpressure applied to an acceleration electrode to form an electron beam.

An element of determining a performance of the FEG includes a brightnessand an energy spread. The brightness indicates an amount of electronbeam as a value indicating how much an electron beam of current can beobtained per unit solid angle, from the electron source per unit area.The energy spread indicates a monochromaticity of an electron beam as arange of wavelengths of the electron beam. Since the FEG can obtain anelectron beam having a high brightness and a narrow energy spreadcompared to other thermionic emission type or Schottky emission electrongun, the FEG is used, as a high resolving power electron gun, for thecharged particle beam apparatus such as a scanning electron microscope(SEM) or a transmission electron microscope (TEM).

As a problem of the FEG, it is known that emission current is unstable.FIG. 1 illustrates a representative time change of current. The sametime change is disclosed in patent document 1. Current emitted from theclean surface of an electron source significantly decreases immediatelyafter the emission and continues its gradual decrease even after thesignificant decrease. Next, noise starts occurring in the current andincreases over time. Then, the current changes to increase and noisefurther increases. A period where a decrease in current is significant,which is indicated by A in FIG. 1, is referred to as a decreasingregion, and a period where the decrease in current is gradual, which isindicated by B in FIG. 1, is referred to as a stable region.

The time change in current of the FEG occurs by adsorbing gas residualin the vacuum on the surface of the electron source. When gas particlesare adsorbed on the clean metal surface, a work function of the surfaceincreases. As a result, the potential barrier of the surface is enlargedand a number of electrons to be emitted decreases. Since the current ofelectron beam decreases, a brightness is deteriorated. Also, since anextraction voltage required to obtain the same current increases, anelectron having a relatively wide energy spread escapes the potentialbarrier whereby the energy spread of the electron beam becomes widened.When a predetermined amount of gas particles are adsorbed on thesurface, a gas adsorbed layer is formed and thus, a change of the workfunction decreases and the current becomes relatively stable. A regionafter the gas adsorbed layer is formed corresponds to the stable region.Even in the stable region, gas is deposited on the adsorbed layer or theadsorbed gas of the surface is substituted with another gas whereby thecurrent gradually keeps decreasing. The gas particles adsorbed on thesurface performs desorption, substitution, or migration for a shortperiod of time, which causes noise in current. Also, positive ionsgenerated by the electron beam collide with the electron source, whichdamages the surface, causing the shape to be uneven, which is alsoregarded as another cause of noise.

The clean metal surface may be obtained again by performing a flashingoperation of heating the electron source for a short period of time. Byheating the electron source at a high temperature, the adsorbed gas ofthe surface is desorbed and metal atoms of the surface migrate, wherebythe surface becomes smooth. Through this, the clean surface can beobtained. As a larger amount of gas is adsorbed on the surface, a highertemperature of flashing is required to clean the surface. In themeantime, the surface of the electron source is melted according to thehigh temperature of flashing and thus, a radius of curvature of a frontend increases. When the radius of curvature increases, a strength of anelectric field to be applied to the surface decreases and thus, anextraction voltage required for the field emission increases.Substantially, there is an upper limit in an extraction voltage to beapplicable and an un-sharpness of diameter of the front end of theelectron source by flashing determines a usage lifespan of the electronsource.

When a user uses the FEG, the user operates an apparatus based on thetime change of current as shown in FIG. 2. The user initially performsflashing 1 of the electron source and then performs increasing 2 of theextraction voltage to thereby emit an electron beam. Since the currentsignificantly decreases in the decreasing region immediately after theemission, the user avoids a use of this region and uses the stableregion after waiting for tens of minutes until the current enters astable region where the decrease in current is gradual. Since thecurrent slowly decreases even during the period of the stable region,the user maintains the current to be greater than a predetermined valueby repeating increasing 2 of the extraction voltage. Every time theextraction voltage is increased, a criterion of an electron opticalsystem changes and thus, the user may need to re-coordinate an opticalaxis. Also, when an observation continues for several hours, noisestarts occurring in the current. The noise disturbs the usage of theFEG. The noise is eliminated by re-flashing the electron source andthereby cleaning the surface. In the case of flashing, the time changeof current is returned to an initial state and thus, the user resumesthe use after waiting until the current enters the stable region.

In general, when performing flashing, the user performs stop 3 of theextraction voltage and stops emitting of the electron beam once. This isbecause when flashing is performed in a state where the extractionvoltage is applied, a protrusion of an atom level is formed on the frontend of the electron source. The phenomenon where the protrusion isformed is referred to as buildup. The buildup occurs when metal atoms ofthe surface melted at a high temperature in flashing are drawn towardsthe front end by the electric field and deposited thereon. Due to theprotrusion, a strength of the electric field concentrated on the frontend increases and emission current increases. However, due to theprotrusion, the adsorbed gas or damage effect is serious and the currenteasily becomes unstable. Due to the above reasons, the buildup isavoided for practical use.

As described above, even though the FEG has a high resolving powercompared to other electron guns, it is inconvenient to use the FEG inthat the time change of current occurs. Also, the time change of currentbecomes an issue with respect to an apparatus requiring a long period ofstable current such as an analysis SEM, a length measurement SEM.Accordingly, it is difficult to apply the FEG. Currently, in many cases,Schottky emission electron gun having a relatively low resolving power,however, having stable current is installed in the above apparatuses.

As a method of automatically keeping maintaining the current of FEG tobe greater than a predetermined magnitude, a method of intermittentlyperforming flashing in the stable region, disclosed in patent document1, or a method of intermittently performing flashing in the decreasingregion, disclosed in patent document 2, are known. In the meantime, as amethod of extending the time change of current to thereby reduce adecreasing speed, a method of enhancing a vacuum degree around theelectron source using a titan sublimation pump and liquid nitrogencooling is disclosed in non-patent document 1. Also, as an electron gunstructure of enhancing the vacuum degree, a charged particle beamapparatus using a non-evaporative getter (NEG) pump is disclosed inpatent document 3, and a charged particle beam apparatus including anNEG pump and an ion pump is disclosed in patent document 4. In addition,as a structure of supplying gas to an electron gun, patent document 5 isproposed.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: U.S. Pat. No. 3,786,268-   Patent Document 2: Japanese Patent Application Laid-Open Publication    No. 2007-73521-   Patent Document 3: Japanese Unexamined Patent Application    Publication No. Hei3 (1991)-505386-   Patent Document 4: Japanese Patent Application Laid-Open Publication    No. 2006-294481-   Patent Document 5: Japanese Patent Application Laid-Open Publication    No. 2007-172862

Non-Patent Document

-   Non-Patent Document 1: B. Cho, Applied Physics Letters, volume    91, (2007) P012105

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

As described above, since emission current of a field emission electrongun (FEG) decreases over time and noise occurs, the FEG is unstable. Asfor one of problems, since current decreases immediately after emission,an electron beam having a high brightness and a narrow energy spread ofan initial emission cannot be stably and continuously used. By using themethod of enhancing the vacuum degree around the electron sourcedisclosed in non-patent document 1, it is possible to extend the timechange and to delay the decreasing speed of current. However, eventhough the vacuum degree is enhanced, the decreasing speed in thedecreasing region is comparatively high and thus, the currentcontinuously decreases at all times. It is difficult to use an electronbeam of this region for the apparatus.

In addition, as another problem, even in the case of the stable regionused in the related art, the current may slowly keep decreasing. Due tothe above, increasing of the extraction voltage and an axis coordinationaccording to the increasing need to be performed once per tens ofminutes. Also, since noise occurs in current over several hours,flashing may be required by stopping the use. Also, to use the stableregion, a waiting time of at least tens of minutes is required. Themethod of continuously providing the stable region by intermittentflashing, disclosed in patent document 1, needs to maintain a statewhere a predetermined amount of gas is adsorbed on the surface of theelectron source. For this, it is difficult to control a flashingstrength. Since the current decrease in the decreasing region isoutstanding, the method of performing intermittent flashing in thedecreasing region needs to perform flashing frequently at short timeintervals in order to maintain the current to be greater than apredetermined value. Frequent flashing makes the front end of theelectron source dull and shortens a lifespan of the electron source.Also, every time flashing is performed, there is a need to stop emittingof the electron beam once. In the case of the method of performingflashing while irradiating the electron beam, disclosed in patentdocument 2, the current becomes unstable due to the protrusion formed onthe front end of the electron source by the buildup. Also, a thermalelectron is generated from a high temperature of electron source duringflashing and noise of the electron beam occurs, which disturbs theusage. The method of enhancing the vacuum degree around the electronsource and extending the time change may smooth a decrease of current inthe stable region and delay a time when noise starts occurring. However,in that the decreasing region is also extended, a waiting time isprolonged in terms of use.

It is an object of the present invention to provide a charged particlebeam apparatus that may stably obtain an electron beam having a highbrightness and a narrow energy spread.

It is another object of the present invention to provide a chargedparticle beam apparatus that may quickly obtain a stable electron beamwhere a decreasing speed is reduced during a long term and noise islittle.

Means for Solving the Problems

The object may be achieved by configuring a charged particle beamapparatus of the present invention including a field emission electronsource and an electrode to apply an electric field to the field emissionelectron source, by including a vacuum exhaust unit to maintain apressure around the field emission electron source to be less than1×10⁻⁸ Pa, by using wherein an electron beam having a central radiationangle within 1×10⁻² sr among electron beams emitted from the fieldemission electron source, and by maintaining a second orderdifferentiation about a time of current of the electron beam to be minusor o for at least one hour after flashing of the field emission electronsource. More preferably, the current of the electron beam having adecreasing rate 10% or less per hour is used.

Also, the object may be achieved by including, in the charged particlebeam apparatus, a heating unit of the field emission electron source,and a detector of current of the electron beam emitted from the fieldemission electron source, and by maintaining the current of the electronbeam emitted from the field emission electron source to be greater thana predetermined value by repeatedly heating the field emission electronsource using the heating unit.

Further, the above object may be achieved by including a heating unitfor the field emission electron source, and by emitting electron beamsfrom the field emission electron source while normally keeping heatingthe field emission electron source in 1500 degrees C. or less.

In addition, the object may be achieved by configuring a chargedparticle beam apparatus including a field emission electron source, anelectrode to apply an electric field to the field emission electronsource, by including a vacuum exhaust unit exhausting around the fieldemission electron source and a gas adsorbed layer forming unit to form agas adsorbed layer on the surface of the field emission electron source,and by controlling a change in current of an electron beam emitted fromthe field emission electron source using the vacuum exhaust unit and thegas adsorbed layer forming unit. More preferably, the vacuum exhaustunit of the charged particle beam apparatus maintains a pressure aroundthe field emission electron source to be less than 1×10⁻⁸ Pa.

Effects of the Invention

According to the present invention, there may be provided a chargedparticle beam apparatus of providing a highly stable electron beam, anda method of controlling the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a time change of emission current of afield emission electron gun (FEG) in the related art.

FIG. 2 is a diagram to describe a change in emission current when usingthe FEG in the related art.

FIG. 3 is a configuration diagram of a scanning electron microscope(SEM) according to a first embodiment.

FIG. 4 is a diagram to describe a time change of emission currentobtainable from the first embodiment.

FIG. 5 is a diagram to describe a change in an emission pattern showinga bright stable region of the first embodiment.

FIG. 6 is a diagram illustrating the time change of emission currentwhen controlling flashing of the first embodiment.

FIG. 7 is a flowchart illustrating an operation when controllingflashing in the first embodiment.

FIG. 8 is a diagram illustrating the time change of emission currentwhen controlling flashing while irradiating the electron beam in thefirst embodiment.

FIG. 9 is a flowchart illustrating an operation when controllingflashing while irradiating the electron beam in the first embodiment.

FIG. 10 is a diagram illustrating the time change when controllingflashing at short time intervals in the first embodiment.

FIG. 11 is a configuration diagram of an SEM including a non-evaporativegetter (NEG) pump valve according to a second embodiment.

FIG. 12 is a diagram illustrating a time change of emission current whencontrolling forming of a gas adsorbed layer in the second embodiment.

FIG. 13 is a flowchart illustrating an operation when controllingforming of the gas adsorbed layer in the second embodiment.

FIG. 14 is a diagram illustrating the time change of emission currentwhen controlling forming of the gas adsorbed layer without irradiatingthe electron beam in the second embodiment.

FIG. 15 is a diagram illustrating the entire configuration of anapparatus including a gas supplier of the second embodiment.

FIG. 16 is a diagram illustrating the entire configuration of anapparatus including a gas adsorbing material of the second embodiment.

FIG. 17 is a diagram illustrating a time change when performing flashingwhile supplying hydrogen gas in a third embodiment.

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, the best mode for carrying out the present invention willbe described by referring to the accompanying drawings. In addition, inthe description of the present specification, “means” is also referredto as “unit” or “device”, for example, a heating means as a heating unitand the like.

First Embodiment

The present embodiment describes a charged particle beam apparatus thatmay stably obtain an electron beam having a high brightness immediatelyafter emission of current and having a narrow energy spread. FIG. 3illustrates a scanning electron microscope (SEM) as a first embodimentof the charged particle beam apparatus. An electron source 4 uses asharpened front end of a single crystal tungsten beam of <310> azimuth.The electron source 4 is fixed to a front end of a tungsten filament 5and is installed within an electron gun chamber 6. The electron gunchamber 6 performs exhaust by an ion pump 20 and a non-evaporativegetter (NEG) pump 23, and is maintained to be less than 1×10⁻⁸ Pa,particularly, less than 1×10⁻⁹ Pa. The NEG pump has advantages in thatit is small, light, and inexpensive compared to other vacuum exhaustmethods such as a method of cooling a titan sublimation pump orapparatus using liquid nitrogen. Even though the NEG pump 23 is heatedby a NEG heating unit 24 once and thereby reaches a room temperature,the NEG pump 23 continues the exhaust. The electron gun chamber 6 isconnected to a first intermediate chamber 7 via an aperture on a centerbetween extraction electrodes 11. Also, the first intermediate chamber 7is connected to a second intermediate chamber 8 via an aperture betweenacceleration electrodes 12. An upper structure above the secondintermediate chamber 8 is generally used as an FEG. The secondintermediate chamber 8 is connected to a sample chamber 9 via anaperture between objective lenses 13. The first intermediate chamber 7performs the exhaust by an ion pump 21, the second intermediate chamber8 performs exhaust by an ion pump 22, and the sample chamber 9 performsexhaust by a turbo molecular pump 25, whereby a differential exhauststructure is achieved.

After flashing the electron source 4 using a flashing power source 16until an adsorbed layer disappears on the surface, an extraction voltageis applied between the electron source 4 and the extraction electrode 11using a high voltage power source 33, and an electron beam 10 is emittedfrom the electron source 4. The electron beam 10 is accelerated by anacceleration voltage applied between the electron source 4 and theacceleration electrode 12 using the high voltage power source 33, andreaches the second intermediate chamber 8. By passing the electron beamthrough an aperture installed in the aperture electrode 31 and therebyeliminate an outer circumferential part, an emission angle of anelectron beam to be used is determined. By connecting a current detector15 to the aperture electrode 31, a change in emission current ismonitored. Monitoring of emission current may be performed by detecting,using the current detector 15, all the current emitted from the electronsource 4. Next, the electron beam 10 is focused by the objective lens 13and irradiated towards a sample 14 disposed on a sample stage 26.Electrons emitted from the sample 14 are detected by an emissionelectron detector 32 and processed by a controller 17, whereby anobserved image is obtained.

Flashing is performed by flowing current into the tungsten filament 5for a predetermined time and by increasing the temperature of theelectron source 4 through current carrying heat. A time for applyingcurrent is maximally a few seconds and applying of the current isperformed several times based on a state of the surface. Prior to usingthe apparatus or in the case of maintenance performed once every fewmonths, a baking operation by heating, using an electron gun heatingunit 30, the electron gun chamber 6 is performed. Through baking, gasemitted from walls of the electron gun chamber 6 is exhausted and theelectron gun chamber 6 may be maintained at the pressure less than1×10⁻⁸ Pa in a normal state. Baking is performed even with respect tothe first intermediate chamber 7 and the second intermediate chamber 8.Since the pressure of the electron gun chamber 6 increases while baking,a rough exhaust valve 27, a rough exhaust valve 28, and a rough exhaustvalve 29 are opened and exhaust gas by using the ion pump 21, the ionpump 22, and the turbo molecular pump 25 in combination. As describedabove, the NEG pump 23 starts exhausting gas by heating with the NEGheating unit 24. However, by heating of the NEG pump 23 while baking, itis possible to effectively exhaust gas temporarily emitted from the NEGpump 23 when the temperature reaches a high temperature, and to preventthe emitted gas from being adsorbed on walls of the electron gun chamber6.

In the case of an observation, the electron source 4 is flashed untilthe gas adsorbed layer disappears on the surface and then, theextraction voltage is applied and the electron beam 10 is irradiated.The current of an emission angle less than 1×10⁻² sr emitted from acenter of the front end of the electron source 4 in the electron beam 10is squeezed by the aperture electrode 31 and is used as probe current.By maintaining the pressure around the electron source to be less than1×10⁻⁸ Pa, the probe current changes, which is different from theconventional time change of FIG. 1, shown in FIG. 4.

In the present embodiment, an observation uses a region where a currentimmediately after emission gradually decreases while maintaining anupwardly convexed high current, as indicated by C in FIG. 4, i.e., aregion where a second order differentiation about time of current isminus or o, and a decreasing rate is less than 10% per hour. Thecontinuity time of this region depends on a vacuum degree around theelectron source and lies in a zone of 10⁻⁹ to 10⁻¹⁰ Pa, whichcorresponds to at least one hour, i.e., from several hours to tens ofhours. Hereinafter, this current region is referred to as a “brightstable region”. The bright stable region corresponds to a new currentregion that has not been known so far. By maintaining the vacuumpressure around the electron source to be less than 1×10⁻⁸ Pa as in thepresent embodiment, and by using the local current of theelectron-beam-center, the bright stable region finally appears and canbe used.

As described above with reference to FIG. 1, in the general time changeof emission current, the current significantly decreases by adsorbinggas on the surface of the electron source. The decreasing speed ofcurrent is greatest immediately after starting the emission, anddecreases over time. The time change of current is downwardly convexedlike a curve shown in the graph of FIG. 1, and a second orderdifferentiation about time of current becomes a plus. As disclosed innon-patent document 1, when enhancing the vacuum degree around theelectron source, the decreasing speed of current decreases. However,same as the related art, the graph of the time change of current draws adownwardly convexed curve and a second order differentiation of currentis a plus. As described above, when the second order differentiationabout time is a plus, the current draws the downwardly convexed curve inthe graph and thus, it is not possible to maintain a high currentimmediately after emission starting.

As described above, by a configuration of enhancing a vacuum degreearound an electron source of the present embodiment and by using thelocal current emitted from a center portion of the clean electron beamsurface, it is possible to draw an upwardly convexed curve in the graphand to use a bright stable region of maintaining the high current. Thebright stable region may be defined as a region where a time immediatelyafter flashing is at least one hour, a decreasing speed of current islow and a second order differentiation about time of current is minus oro. The bright stable region had not been reported before and could notbe recognized by the conventional vacuum degree.

An emission pattern in the bright stable region according to the presentembodiment will be described with reference to FIG. 5. In an upperportion of FIG. 5, emission pattern obtainment times (circled numbers 1to 8) in the time change of the same probe current as FIG. 4 areindicated. In a lower portion of FIG. 5, an emission pattern of eachobtainment time is indicated. The emission pattern corresponds to animage acquired by lighting an electron emitted from an electron sourceagainst a fluorescent screen and an emission part of electron of theelectron source surface appears in correspondence to a brightness of thepattern. It can be known from FIG. 5 that the emission pattern becomesdark from an external circumferential part over time (circled numbers 1to 8) and a center portion maintains a brightness. That is, FIG. 5 showsthat the gas adsorption starts from the outer circumferential part ofthe front end of the electron source, the gas adsorption does not occurin a center part while it is ongoing, and a time difference exists untilthe adsorption of the center part starts. From the result of the abovepattern observation, it is possible to explain that the bright stableregion appears in the local current of the center part of the electronbeam. The range of the electron beam where the bright stable regionappears corresponds to a local electron beam emitted from a regionbetween surface <310> of electron source surface center and intermediatesurface <410> of surface <100> that is closest to the surface <310> andto which the electron beam is not emitted. An open angle of bothsurfaces is 77 mrad from an inner product of a normal vector, andcorresponds to an electron beam emitted at a solid angle within about1×10⁻² sr. When using a local electron beam at a further center part,the continuity time of the bright stable region is extended and it ispreferable to use an electron beam of a solid angle within 1×10⁻³ srexperimentally.

The electron beam of the bright stable region is emitted from thesurface of which a work function of the clean electron source is low,and thus has a high brightness and a narrow energy spread compared tothe conventional stable region. In addition, the decreasing speed ofcurrent further decreases and noise is also little. Also, since thesecond order differentiation about time of current is minus, the highcurrent immediately after emission is mostly maintained in apredetermined state for a long period of time. By using the current ofthis region for an SEM observation, a stable observation having a highresolving power compared to the related art may be performed. Also,since the continuity time of the bright stable region is from severalhours to tens of hours, the continuity time is sufficiently long withrespect to a one-time use time of the apparatus in a general SEM and thelike for scientific analysis. Accordingly, since increasing of theextraction voltage, the axis coordination of the optical systemaccording to the increasing, and re-flashing of the electron source arenot required for the one-time observation, there is no need to stop theobservation. Also, since a waiting time required to resume theobservation in the related art is not required, a user may start theobservation immediately after initial flashing. In addition, since theextraction voltage is constant during the observation, the axiscoordination of the optical system performed when the accelerationvoltage is changed becomes easy. Since a flashing frequency decreases, alifespan of the electron source is extended.

By repeating flashing in the bright stable region, the bright stableregion may be consistently used. The time change of current in this caseis shown in FIG. 6 and a flowchart of an operation is shown in FIG. 7.In FIGS. 6, 1, 2, and 3 indicate flashing, increasing of the extractionvoltage, and stop of the extraction voltage, respectively.

In FIG. 7, when an operation starts by a predetermined manipulation, theextraction voltage increases (72) by predetermined current afterflashing (71). Next, a normal operation (73) is performed. Whether thecurrent has reached predetermined timing of re-flashing is determined(74) at predetermined time intervals. When timing of re-flashing isreached, re-flashing is performed by stopping (75) the extractionvoltage.

Also, since an amount of gas adsorbed on the electron source surface issmall in the bright stable region, the surface may be cleaned byflashing of a temperature less than 2000 degrees C. in the related art.In particular, as flashing is performed at the low temperature of lessthan 1500 degrees C., a buildup barely occurs even in a state where theextraction voltage is applied, and an amount of thermal electrons occursis small. Accordingly, flashing may be performed in a state where theobservation is ongoing without stopping irradiation of electron beams.The time change of current in this case is shown in FIG. 8 and aflowchart thereof is shown in FIG. 9. In FIG. 8 and FIG. 9, the samenumerical numbers as used in FIG. 6 and FIG. 7 refer to the same objectsand the same steps. In FIG. 9, 76 indicates flashing (low temperatureflashing) less than 1500 degrees C.

When flashing is performed while the observation is ongoing, it ispossible to prevent noise occurring in current during flashing fromaffecting the observation and enhance the user convenience by performingflashing when scanning of an electron beam goes round a sample surface,or by stopping obtaining of the observed image for only a few seconds offlashing.

The aforementioned observation method using flashing may always obtainan electron beam having a high brightness and a narrow energy spreadcompared to a method of performing flashing in the decreasing region ofcurrent disclosed in patent document 2. Also, since the decreasing speedof current is slow and the second order differentiation about time ofcurrent is minus, i.e., makes a change expressed as an upwardly convexedgraph, it is possible to obtain a stable high current for a long periodof time even though a time interval of flashing is extended.

Also, since the low temperature flashing is performed, the buildupbarely occurs and an amount of thermal electrons occurring decreases.Since it is difficult to melt the electron source, the lifespan of theelectron source is further extended. Since the electron source does notbecome dull, the extraction voltage may also be used as a predeterminedvalue for a long period of time. Also, since it is possible to use anelectron source of another material that cannot be used in 2000 degreesC. of flashing in the related art, and having a melting point less thantungsten.

The user may perform flashing at random timings by giving an instructionusing a manipulator 19 of the apparatus of the present embodiment shownin FIG. 3. Also, the user may select whether to perform flashing afterstopping applying of extraction voltage and stopping the observationonce, or whether to perform flashing during the observation. Theflashing strength may be variably selected, and may be appropriatelyused depending on an amount of gas adsorbed on the surface of theelectron source. By displaying the current monitored by the currentdetector 15 on a display 18, the user may easily estimate timing offlashing.

Also, the timing of flashing may be automatically determined by acontroller 17 and be informed to the user by displaying the timing onthe display 18. As one of criteria to determine the timing, as shown inFIG. 6, when current I_((t)) detected by the current detector 15 becomesto be less than value αI_((o)) obtained by multiplyingemission-immediately-after current I_((o)) by a predetermined value α,flashing is performed. Typically, by using a of 0.8 and more, the usermay use the apparatus without recognizing the deterioration in thebrightness of the observed image. In particular, when α is greater than0.95, the variation width of current becomes narrow as shown in FIG. 10,mostly consistent current may be maintained, and the brightness of theobserved image is consistent.

As another criterion, as shown in FIG. 6, when a decreasing ratio[I_((t))−I_((t+tc))]/I_((t)) of current per time interval t_(c) becomesto be greater than a predetermined value β, flashing is performed. Inconsideration of the time change of current in the bright stable region,when t_(c) is less than 60 minutes and β is greater than 0.01, thechange in current may be easily recognized. In particular, when0.05≧β≧0.01, the current may be consistently maintained as shown in FIG.10.

As still another criterion, when a predetermined time t₁ is elapsedafter flashing, flashing is performed again. Typically, when t₁ isgreater than 1 hour, a probability of disturbing the observation of theuser decreases. In the meantime, when t₁ is set to be less than 1 hour,it is possible to consistently maintain the current by decreasing thevariation width of current.

Instead of the current value I_((t)) used for the above determination,the average current value of a predetermined period may be used. In thiscase, determination may be made regardless of the effect of noiseoccurring in current. Also, when determining the timing based on theentire current, values such as α, β, and t_(c), may be slightlydifferent, however, may be determined based on the same criterion.

By using the aforementioned determination criterion and automatingflashing using the controller 17, the user's manipulation is notrequired and the bright stable region is continued. Through this, it ispossible to enhance the user convenience. Also, the automation issuitable for an SEM requiring a long term observation such as ananalysis SEM, or a critical dimension SEM needing a long term unmannedoperation in an inline inspection of a semiconductor manufacturingfactory and the like. It is possible to realize the apparatus having ahigh resolving power by applying the present embodiment.

Also, timings such as when exchanging the sample 14 of FIG. 3, or whensignificantly moving an observation location of a sample, are added inapplying the present embodiment, in addition to the flashing timing. Theflashing automation may be selected using the manipulator 19. Also,whether the flashing is being automated is displayed on the display 18.Substantially, it is possible to inform the user by displaying theflashing on the display 18 for a few seconds where flashing isperformed.

By repeating flashing in the bright stable region and automaticallyadjusting, using the controller 17, the extraction voltage according toa slight decrease in current, it is possible to consistently maintainthe current. In this case, a voltage may slightly increase and afrequency may be low. Even though the axis coordination of the opticalsystem is required according to the voltage increase, this coordinationis also performed by the controller 17.

As another usage method of the present embodiment, the continuity timeof the bright stable region is extended by normally keeping heating theelectron source 4 by means of the flashing power source 16. This isbecause a probability that gas is adsorbed on the surface decreases asthe temperature of the electron source increases. As the bright stableregion continues for a long period of time, increasing of the extractionvoltage or frequency of re-flashing decreases. In the case of normalheating, by setting the heating temperature to be less than 1500 degreesC., particularly, 100 degrees C. to 100 degrees C., the buildup oremitting of thermal electron does occur and the stable current may beobtained. Also, when the electron source is heated at 1500 degrees C. ormore, a protrusion is formed on the front end of the electron source dueto the buildup. However, since the vacuum degree is enhanced, the changein current may decrease.

Even though tungsten of <310> crystal azimuth is used to the electronsource in the present embodiment, the same effect may be achieved evenin other crystal azimuths, for example, a low work function of <111> andthe like. Also, the same effect may be achieved in the electron sourceusing the same field emission. Other materials such as LaB₆, a carbonfiber may be used for the electron source. Also, in addition to thecurrent carrying heating method described in the present embodiment, theflashing method may be replaceable as far as a corresponding method mayeliminate gas adsorbed on the surface of the electron source. Forexample, the flashing method may be performed even in a method ofinstalling, within the electron gun chamber 6, a new filament emittingthermal electrons, and irradiating thermal electrons from this filamenttowards the electron source 4, or an electric field evaporating method.As another method, the flashing method may be performed in a method ofinstalling a light source such as a laser and irradiating a beam towardsthe electron source 4, or a method of installing a gas supplier withinthe electron gun chamber 6 and irradiating rare gas of hydrogen, helium,argon, and the like towards the electron source 4.

According to the aforementioned present embodiment, it is possible tostably obtain an electron beam having a high brightness and a narrowenergy spread. By repeating flashing and decreasing the heatingtemperature, it is possible to continuously use the bright stable regionwithout stopping the observation. It is possible to provide a chargedparticle beam apparatus that may perform a high resolving powerobservation for a long period of time by using the electron beam.

Second Embodiment

Hereinafter, a second embodiment will be described with reference toFIG. 11 to FIG. 16. The present embodiment describes a charged particlebeam apparatus of including a gas adsorbed layer forming unit anddecreasing a time of a decreasing region to thereby obtain the stablecurrent of a stable region compared to a related art.

FIG. 11 illustrates the entire configuration of an SEM that is thecharged particle beam apparatus according to the second embodiment. Thetime change of emission current in the present embodiment is shown inFIG. 12, and a flowchart of an operation is shown in FIG. 13. In FIG.13, 77 indicates a process of forming a gas adsorbed layer and the samenumbers as used in FIG. 7 indicate the same process.

The configuration of the apparatus is mostly the same as the firstembodiment, however, has a configuration of connecting the NEG pump 23to the electron gun chamber 6 by an NEG pump valve 34 disposed as a gasadsorbed layer forming unit. In the present embodiment, the electronsource 4 is flashed using the flashing power source 16 until the gasadsorbed layer disappears on the surface and then the electron beam 10is irradiated by applying the extraction voltage to the extractionelectrode 11 using the high voltage power source 33. Only during aperiod of the decreasing region immediately after emission, only a halfof the NEG pump valve 34 is opened or is closed using the controller 17to thereby decrease the exhaust speed of the NEG pump 23 and totemporarily increase the vacuum pressure of the electron gun chamber 6.At the same time, it is possible to accelerate the gas adsorption bydecreasing the exhaust speed of the ion pump 20. The pressure of theelectron gun chamber in this case is typically from 1×10⁻⁸ to 1×10⁴ Pa,more preferably, from 1×10⁻⁷ Pa to 1×10⁻⁵ Pa. Due to the pressureincrease, the gas adsorbed layer is formed on the surface of theelectron source for a short period of time and the current enters thestable region. Next, the NEG pump valve 34 is opened and the pressure ofthe electron gun chamber 6 is maintained again to be less than 1×10⁻⁸Pa, particularly, 1×10⁻⁹ Pa. After making the current reach apredetermined magnitude by increasing again the extraction voltage, theobservation in the stable region is performed. When noise in currentincreases, irradiating of the electron beam is stopped and then flashingis performed. After flashing, irradiating of the electron beam andforming of the gas adsorbed layer is again performed and the current ofthe stable region is again obtained.

According to the present embodiment, it is possible to use the currentof the stable region at a short waiting time. Also, since the vacuumpressure around the electron source is low in the stable region comparedto the related art, it is possible to obtain the current of the stableregion of which the decreasing speed further decreases, of which noiseis low, and of which the continuity time is long. The continuity time ofthe stable region is approximately from tens of hours to hundreds ofhours in 10⁻⁹ to 1×10⁻¹⁰ Pa and is longer than several hours in therelated art. Since the decreasing speed decreases, the high current ismaintained for a long period of time and an opportunity of increasingthe extraction voltage decreases. Also, since a period where noiseoccurs in current is extended, a flashing frequency decreases. Asdescribed above, a number of manipulations required for stopping theobservation decreases and thus, the user convenience is enhanced. Also,a decrease in the flashing frequency extends the lifespan of theelectron source.

The user may perform flashing at random timings and forming of the gasadsorbed layer by giving an instruction using the manipulator 19. When aperiod of gas adsorption is extended, gas is excessively adsorbed on thesurface of the electron source. Thus, the controller 17 determine atiming for terminating the gas adsorption and the determined timing maybe informed to the user by displaying the determined timing on thedisplay 18. Also, the user may select whether to automatically form thegas adsorbed layer using the controller 17. In the case of theautomation, the gas adsorption starts immediately after flashing and isterminated at the timing determined by the controller 17.

As a criterion of the timing for terminating the gas adsorption, the gasadsorption is terminated when current I_((t)) detected by the currentdetector 15 becomes to be less than value ΔI_((o)) obtained bymultiplying the current I_((o)) immediately after emission by apredetermined value γ. Typically, by using γ of less than 0.2, it ispossible to obtain the current having a low decreasing speed. Inparticular, by using γ of less than 0.1, it is possible to obtain thecurrent having a further low decreasing speed.

As another criterion, as shown in FIG. 12, when a decreasing ratio[I_((t))−I_((t+ts))]/I_((t)) of current per time interval t_(s) becomesto be less than a predetermined value ε, the gas adsorption isterminated. Typically, when t_(s) is less than 5 minutes, evaluation iseasy and in this case, ε may be less than 0.05.

As still another criterion, when a predetermined time t_(i) is elapsedafter starting the gas absorption, the gas absorption is terminated.Typically, when t_(i) is less than ten minutes, the gas adsorbed layeris formed on the surface. In addition, the above method may be appliedwithout irradiating the electron beam. In this case, the time change ofcurrent appears as shown in FIG. 14, and forming of the gas adsorbedlayer is terminated. After the pressure of the electron gun chamber 6becomes to be less than 1×10⁻⁸ Pa, emitting of the electron beam 10starts.

When a predetermined time is elapsed after entering the stable region,noise starts occurring in the current. In this case, there is a need toclean the surface of the electron source by re-flashing. Even thoughflashing may be performed by the user at random timings similar to thefirst embodiment, the timing may be determined by the controller 17 andthe determined timing may be informed to the user by displaying thedetermined timing on the display 18. Also, flashing automation using thecontroller 17 may be selected by the manipulator 19.

As one criterion for determining the timing, with respect to the currentper time t_(n) of FIG. 12, when a change ratio(I_(max)−I_(min))/[(I_(max)+I_(min))/2] obtained by dividing adifference (I_(max)−I_(min)) between a maximum value I_(max) of currentand a minimum value I_(min) of current by the average becomes to begreater than a predetermined value ξ, the timing is determined. As acriterion of when the observed image does not become worse, a criterionwhere t_(n) is less than 5 minutes and ζ is less than 0.1 is typicallyused.

As another criterion, it corresponds to a case where a time t₂ iselapsed after flashing. In consideration of the continuity time of thestable region, t₂ is typically several days and more.

The timing for terminating the gas absorption and the flashing timingmay be determined using all the current. Even though values such as γ,ε, t_(s), t_(i), etc., may slightly vary, the determination may beperformed alike.

Since the controller 17 automatically performs forming of the gasadsorbed layer and re-flashing, the user's manipulation is not requiredand it is possible to continuously use only the stable region based onseveral days or several months.

Also, it is possible to consistently maintain the current byautomatically adjusting the extraction voltage using the controller 17according to a slight decrease in current. In this case, a voltage mayslightly increase and a frequency may be low. Even though the axiscoordination of the optical system is required according to the voltageincrease, this coordination is also performed by the controller 17. Itis possible to enhance the user convenience according to the automation.Also, an FEG may be applicable to an SEM requiring a long termobservation such as an analysis SEM, or a length measurement SEM usedfor an inline inspection of a semiconductor manufacturing factory andthe like. Accordingly, it is possible to achieve an apparatus having ahigh resolution compared to the related art.

In addition to the above, a timing for exchanging a sample, a regularmaintenance of the SEM, and the like are added to the timing for appliedre-flashing and performing the gas absorption. Whether the automation iscurrently ongoing is displayed on the display 18 and thereby is informedto the user. Also, while substantially performing flashing or gasabsorption, it is displayed on the display 18.

In the present embodiment, the current of the stable region is obtainedfor a relatively short waiting time by decreasing the exhaust speed ofthe vacuum exhaust unit and by forming the gas adsorbed layer on thesurface of the electron source during the period for the decreasingregion. As shown in FIG. 15, it is possible to accelerate forming of thegas adsorbed layer by installing the gas supplier 35 in the electron gunchamber 6 and spraying gas towards the surface of the electron source.

The gas supplier 35 is used as follows. The electron source 4 is cleanedby flashing and then, the electron beam 10 is irradiated. Gas such ashydrogen, oxygen, carbon dioxide, or methane, etc., is supplied from thegas supplier 35 towards the electron source 4 and is adsorbed on thesurface. A gas supply method includes a method of continuously supplyinga predetermined amount of gas and a method of intermittently supplyinggas. By decreasing the exhaust speed of the NEG pump 23 or the ion pump20 at the same time with the gas supply, it is possible to accelerateadsorbing of the gas. In the meantime, without decreasing the exhaustspeed, it is possible to selectively form, as the gas adsorbed layer onthe surface of the electron source, the gas supplied from the gassupplier 35. By supplying of the gas, the gas adsorbed layer is formedon the surface of the electron source 4 and the current quicklydecreases and thereby the current enters the stable region. Next,supplying of the gas is stopped and the electron gun chamber ismaintained to be less than 1×10⁻⁸ Pa, particularly, 1×10⁻⁹ Pa. Afterincreasing emission current by performing increasing 2 of the extractionvoltage, the observation in the stable region is performed. When noisestarts occurring in the current, applying of the extraction voltage isstopped and irradiation of the electron beam is stopped and thenflashing is performed. After flashing, irradiation of the electron beamand forming of the gas adsorbed layer are again performed and then, thecurrent of the stable region is again obtained.

A period and timing for supplying the gas is the same as theaforementioned period and timing for forming the gas adsorbed layer andthe automation may be performed using the controller 17 according toflashing. Also, the user may discharge a predetermined amount of gas atrandom timings using the manipulator 19.

As another example of a means of spraying gas towards the other electronsource, as shown in FIG. 16, there is a method that may accelerateforming of the gas adsorbed layer by installing a gas adsorbing material36 in the electron gun chamber 6 and spraying gas towards the surface ofthe electron source.

Like the gas supplier 35, the gas adsorbing material 36 is used toaccelerate forming of the gas adsorbed layer by discharging gas from aninside. The gas adsorbing material 36 is a material to store gas ofhydrogen and the like within the inside, and discharges the stored gasby performing heat processing and the like. Instead of installing thegas adsorbing material 36, it is possible to heat the NEG pump 23 and todischarge the gas from the NEG pump 23. Even though the NEG pump 23 is avacuum pump containing hydrogen, the NEG pump 23 discharges the hydrogenby heating. By controlling the heating unit 23 using the controller 17,it is possible to form the gas adsorbed layer using only the NEG pump23.

As still another example of a means of spraying gas towards the otherelectron source, there is a method that may form the gas adsorbed layerby heating a part within the electron gun or a portion thereof, and byaccelerating discharging of the gas from the heated part.

The period and timing for discharging the gas is the same as theaforementioned period and timing for forming the gas adsorbed layer andthe automation may be performed using the controller 17 according toflashing. Also, the user may discharge a predetermined amount of gas atrandom timings using the manipulator 19.

By installing the gas supplier 35 or the gas adsorbing material 36, alarge amount of gas exists in a local part close to the electron sourceand thus, it is possible to form the adsorbed layer on the surface ofthe electron source for a relatively short period of time. Also, it ispossible to form the adsorbed layer while suppressing the entirepressure increase in the electron gun chamber. Also, composition of thegas adsorbed layer on the surface may be selected by supplying onlyspecific gas.

According to the present embodiment, compared to the related art, it ispossible to obtain an electron beam having a low decreasing speed andlittle noise in a relatively short waiting time. By using this electronbeam, there may be provided the charged particle beam apparatus that maystabilize the current for a long period of time and decrease a frequencyof increasing of the extraction voltage or a flashing frequency.

Third Embodiment

Hereinafter, a third embodiment will be described with reference to FIG.17.

The third embodiment describes a charged particle beam apparatus thatmay spray hydrogen gas during flashing of an electron source, and cleanthe surface of the electron source by low temperature flashing. In anSEM including the gas supplier 35 or the gas adsorbing material 36described in the second embodiment, the electron source 4 may becleansed by flashing at a temperature lower than the related art,typically, less than 1800 degrees C., limitedly less than 1500 degreesC., by flashing performed while spraying hydrogen towards the electronsource 4. This is because the supplied hydrogen chemically activatescarbon-based adsorbed gas that could not be eliminated if it was notheated at the temperature of about 2000 degrees C. in the related art,to thereby eliminate the carbon-based adsorbed gas from the surface evenin the low temperature.

Since the heating temperature in flashing decreases, the front end ofthe electron source may not easily become dull and thus, the lifespan ofthe electron source is extended. Also, even the extraction voltage whenemitting the electron beam may be fixed, and the axis coordination ofthe optical system is not required. Also, even though only an electronsource of tungsten and the like having a high melting point was used dueto a high temperature of flashing, low temperature flashing is enabledand thus, it is possible to use an electron source of a material havinga low melting point.

By applying the present embodiment to the method of continuing thebright stable region, described in the first embodiment, by flashing, itis possible to more effectively prevent a front end of an electronsource from becoming dull. In this case, the time change of current isshown in FIG. 17. As described above, in the bright stable region, it ispossible to continuously use the bright stable region while continuingthe observation by repeating flashing at the low temperature of lessthan 1500 degrees C. When a long period of time is elapsed, carbon-basedgas starts being adsorbed on the surface of the electron source. Thecarbon-based adsorbed gas cannot be eliminated by flashing of 1500degrees C. In this case, by stopping the observation once, supplyinghydrogen gas to the electron source, and performing flashing at lessthan 1500 degrees C., it is possible to eliminate the carbon-based gas.Since flashing of the electron source may be performed at thetemperature of less than 1500 degrees C. at all times, it is possible tosuppress dullness of the front end of the electron source.

An example applied to an SEM is described in the first embodiment to thethird embodiment. However, even though the present invention is appliedto another charged particle beam apparatus such as an FEG, atransmission electron microscope (TEM), a scanning transmission electronmicroscope, an analysis apparatus, it is possible to obtain the sameeffect.

Major points of the present invention, in addition to disclosed inclaims, will be briefly described as follows.

A charged particle beam apparatus includes a controller of a heatingunit in which when current of electron beam becomes αI_((o)) withrespect to initial value I_((o)) of current I_((t)) of the electron beamemitted from a field emission electron source, heating of the fieldemission electron source is performed and α≧0.95.

A heating temperature of the field emission electron source of thecharged particle beam apparatus is less than 1500 degrees C.

A charged particle beam apparatus includes a heating unit of a fieldemission electron source in which an electron beam is emitted from thefield emission electron source while normally keeping heating the fieldemission electron source at the temperature from 100 degrees C. to 1000degrees C.

A charged particle beam apparatus includes a controller of a heatingunit in which a field emission electron source is heated again everytime a predetermined time is elapsed after heating the field emissionelectron source.

A charged particle beam apparatus includes a controller of a gasadsorbed layer forming unit and a current detector of an electron beamemitted from a field emission electron source in which when current ofelectron beam becomes γI_((o)) with respect to initial value I_((o)) ofcurrent I_((t)) of the electron beam emitted from the field emissionelectron source, forming of the gas adsorbed layer on the surface of thefield emission electron source is terminated and γ≦0.2, particularly,γ≦0.1.

A charged particle beam apparatus includes a controller of a gasadsorbed layer forming unit and a current detector of an electron beamemitted from a field emission electron source in which when a decreasingratio [I_((t))−I_((t+ts))]/I_((t)) of current per time interval isbecomes to be greater than a predetermined value ε with respect tocurrent I_((t)) of the electron beam emitted from the field emissionelectron source, forming of the gas adsorbed layer on the field emissionelectron source is terminated and t_(s)≦5 minutes, and ε≦0.05.

A charged particle beam apparatus includes a controller of a gasadsorbed layer forming unit in which forming of a gas adsorbed layer ona field emission electron source when a predetermined time is elapsedafter heating the field emission electron source is terminated.

INDUSTRIAL APPLICABILITY

The present invention relates to a charged particle beam apparatus suchas an electron microscope including a field emission electron gun, andmore particularly, to a charged particle beam apparatus for stabilizingthe current of an electron beam and a method of controlling the same.

EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS

-   1 . . . FLASHING-   2 . . . INCREASING OF EXTRACTION VOLTAGE-   3 . . . STOP OF EXTRACTION VOLTAGE-   4 . . . ELECTRON SOURCE-   5 . . . FILAMENT-   6 . . . ELECTRON GUN CHAMBER-   7 . . . FIRST INTERMEDIATE CHAMBER-   8 . . . SECOND INTERMEDIATE CHAMBER-   9 . . . SAMPLE CHAMBER-   10 . . . ELECTRON BEAM-   11 . . . EXTRACTION ELECTRODE-   12 . . . ACCELERATION ELECTRODE-   13 . . . OBJECTIVE LENS-   14 . . . SAMPLE-   15 . . . CURRENT DETECTOR-   16 . . . FLASHING POWER SOURCE-   17 . . . CONTROLLER-   18 . . . DISPLAY-   19 . . . MANIPULATOR-   20 . . . ION PUMP-   21, 22 . . . ION PUMPS-   23 . . . NON-EVAPORATIVE GETTER (NEG) PUMP-   24 . . . NEG HEATING UNIT-   25 . . . TURBO MOLECULAR PUMP-   26 . . . SAMPLE STAGE-   27, 28, 29 . . . ROUGH EXHAUST VALVES-   30 . . . ELECTRON GUN HEATING UNIT-   31 . . . APERTURE ELECTRODE-   32 . . . EMISSION ELECTRON DETECTOR-   33 . . . HIGH voltage POWER SOURCE-   34 . . . NEG PUMP VALVE-   35 . . . GAS SUPPLIER-   36 . . . GAS ADSORBING MATERIAL

What we claim is:
 1. A charged particle beam apparatus including a fieldemission electron source, an electrode to apply an electric field to thefield emission electron source, and a vacuum exhaust unit formaintaining a pressure around the field emission electron source usinglow vacuum, the apparatus comprising: a gas adsorbed layer forming unitfor forming a gas adsorbed layer on the surface of the field emissionelectron source, wherein a change in current of an electron beam emittedfrom the field emission electron source is controlled using the gasadsorbed layer forming unit, wherein the vacuum exhaust unit includes anon-evaporative getter pump, a valve of which a conductance is variablein an upstream side.
 2. The charged particle beam apparatus according toclaim 1, wherein the vacuum exhaust unit maintains the pressure aroundthe field emission electron source to be less than 1×10⁻⁸ Pa.
 3. Thecharged particle beam apparatus according to claim 2, wherein the gasadsorbed layer forming unit is configured to form the gas adsorbed layeron the field emission electron source by changing an exhaust speed ofthe vacuum exhaust unit and increasing the pressure around the fieldemission electron source.
 4. The charged particle beam apparatusaccording to claim 3, wherein the vacuum exhaust unit is configured tobe capable of changing the exhaust speed.
 5. The charged particle beamapparatus according to claim 2, comprising: a controller of the gasadsorbed layer forming unit; a manipulator for selecting whether toautomatically performing heating of the field emission electron sourceusing the controller; and a display for displaying a result selected bythe manipulator.
 6. The charged particle beam apparatus according toclaim 2, wherein the vacuum exhaust unit maintains the pressure aroundthe field emission electron source to be less than 1×10⁻⁹ Pa, whenobservation of a sample is performed.
 7. The charged particle beamapparatus including a field emission electron source, an electrode toapply an electric field to the field emission electron source, and avacuum exhaust unit for maintaining a pressure around the field emissionelectron source using low vacuum, the apparatus comprising: a gasadsorbed layer forming unit for forming a gas adsorbed layer on thesurface of the field emission electron source, wherein a change incurrent of an electron beam emitted from the field emission electronsource is controlled using the gas adsorbed layer forming unit, wherein:the gas adsorbed layer forming unit includes a gas supplier forsupplying gas from an outside of the apparatus, and forming the gasadsorbed layer on the surface of the field emission electron source byspraying the gas from the gas supplier towards the field emissionelectron source.
 8. The charged particle beam apparatus according toclaim 7, comprising: a heating unit of the field emission electronsource, wherein the field emission electron source is heated whilesupplying the gas from the gas adsorbed layer forming unit towards thefield emission electron source.
 9. The charged particle beam apparatusaccording to claim 8, wherein a heating temperature of the fieldemission electron source is less than 2000 degrees C.
 10. The chargedparticle beam apparatus according to claim 7, wherein the vacuum exhaustunit maintains the pressure around the field emission electron source tobe less than 1×10⁻⁸ Pa.
 11. The charged particle beam apparatusaccording to claim 10, wherein the gas adsorbed layer forming unit formsthe gas adsorbed layer on the field emission electron source by changingan exhaust speed of the vacuum exhaust unit and increasing the pressurearound the field emission electron source.
 12. The charged particle beamapparatus including a field emission electron source, an electrode toapply an electric field to the field emission electron source, and avacuum exhaust unit for maintaining a pressure around the field emissionelectron source using low vacuum, the apparatus comprising: a gasadsorbed layer forming unit for forming a gas adsorbed layer on thesurface of the field emission electron source, wherein a change incurrent of an electron beam emitted from the field emission electronsource is controlled using the gas adsorbed layer forming unit, whereinthe gas adsorbed layer forming unit includes a gas adsorbing material inthe charged particle beam apparatus, and forms the gas adsorbed layer onthe surface of the field emission electron source by spraying the gasfrom the gas adsorbing material towards the field emission electronsource.
 13. The charged particle beam apparatus according to claim 12,wherein the vacuum exhaust unit maintains the pressure around the fieldemission electron source to be less than 1×10⁻⁸ Pa.
 14. The chargedparticle beam apparatus according to claim 13, wherein the gas adsorbedlayer forming unit forms the gas adsorbed layer on the field emissionelectron source by changing an exhaust speed of the vacuum exhaust unitand increasing the pressure around the field emission electron source.